|This tip/technique may seem blindingly obvious to some, but I wish I had known about it before this summer, it would have saved me so much time and trouble.
If you have to sew a corner - on a shirt collar, a pocket, or as I just did, the front of a waistcoat which has three corners when attaching the facing, don't sew to (what you hope is) 5/8" of the corner point and pivot.
Instead, sew right off the edge. Cut threads and re-present the sewing to the machine at the (now swivelled) edge, and re-start sewing at the raw edge. Everything will line up, again, at 5/8".
You were going to trim that corner sewing anyway: this way you have two lines of machine stitching, both at exactly 5/8", which help you to cut the corner point off.
Result: accurate seam lines, plus two lines of stitching at the vital corner-turning point.